Indigo dyestuff is a member of the group of the vat dyes; it is insoluble in water and for the purpose of dyeing it must be brought into a fiber-reactive, water-soluble form with the aid of alkali and reduction agents which, when oxidized by means of atmospheric oxygen, provides indigo blue. Other vat dyes, for example indanthrene and sulfur dyestuffs, are also understood by the term "indigo dyestuffs".
A method as mentioned above is described in German Patent Publication DE 43 42 313 A1 for use with warp yam sheets. A relatively small vat is provided with this known method for applying the dyestuffs, a first wet dwelling section being connected downstream of the vat prior to squeezing, i.e. mechanical dewatering, with a low-oxygen atmosphere therefore to effectively function in relation to the diffusion of the dyestuff into the interior of the individual fibers as an extension of the vat. The substrate is moved at full conveying speed in the wet-dwelling section, which is therefore called a dynamic dwelling section.
It is possible to use installations in connection with indigo dyeing, in particular when dyeing warp yarn sheets, which in their basic structure are similar to sizing machines. A sizing machine with an extremely short steeping bath is described in German Patent Publication DE 44 37 704 A1. In this known machine, the lower half of the circumference of one squeezing roller of a squeezing unit serves as a sizing contact section. A steeping vat is not required with this known machine.
A problem when dyeing with indigo is in that the colorations obtained often have insufficient crock fastness, or require extensive rinsing sections, because of the basically low bath and liquor exhaustion. The (reduced) dye remnant which has remained in the liquor can begin to oxidize there already. In the course of the operation, this oxidized dyestuff is also deposited on the textile substrate. However, the previously oxidized dyestuff can no longer assist with real dyeing, since it can no longer be fixed on the fibers.
In order to overcome this problem, it is proposed in the unpublished German Patent document DE 196 28 806 A1, to perform dyeing, or respectively the absorption of the reduced indigo dyestuff by the fiber, in the presence of high electrolyte concentrations, and to maintain the high electrolyte concentration up to the reoxidation of the reduced dyestuff (for forming the-dye pigment). This can be achieved in that the substrate soaked with the steeping liquor is brought to the oxidation process without prior rinsing, since the substrate received after squeezing the liquor out still contains sufficient amounts of electrolyte. The known method makes possible the continuous dyeing of cellulose-containing textiles with indigo in a single operation, in that the reduced indigo is absorbed by the textile material in the presence of an aqueous liquor as the electrolyte, containing reduction agents, alkali, and in addition a further dissolved alkali metal salt at a concentration between 200 and 350 grams per liter (g/l), whose pH value has been set between 10.2 and 11.3, then the reduced indigo absorbed by the fibers is again oxidized into a pigment while maintaining the high electrolyte concentration, and dyeing is finished in a customary manner.
If the method in accordance with German Patent Document DE 196 28 806 A1 were combined with the method in accordance with German Patent Publication DE 43 42 313 A1--possibly while employing the principles of German Patent Document DE 44 37 704 A1--good dyestuff yields would result, but a considerable portion of the dyestuff used would continue to reach the rinsing bath downstream of the oxidation process.